Google Reader implemented PuSH:Pubsubhubbub (worst protocol name ever!). Most blog platforms ping using PuSH, so the update needs no polling and is thus almost instantaneous.
It handles all the "dirty" parts of feed reading (checking for new items, normalizing fields) and adds near instant updates for sites that ping hubs.
When I finally get around to writing my own feed reader, I definitely plan on using it.
I wonder how hard this would be to get it implemented in some of the other readers.